1. Field of the Invention
The present invention concerns a variable attenuator device for a rectangular waveguide.
2. Description of the Related Art
Known attenuators are in general produced using PIN diodes arranged on a printed-circuit substrate in a .pi. or T arrangement, and connected by microstrip lines. The diodes are used as controlled resistors to absorb the microwave variably, with good performance in terms of the reproducibility of the various attenuation values which can be selected in this way, since the resistor properties of PIN diodes are fairly stable. However, the microwave propagating in the guide must undergo a guide/line transition upstream of the attenuator and a line/guide transition downstream of it (relative to the propagation direction of waves in the guide), and this transition may give rise to matching problems. Above all, an attenuator device of this type exhibits insertion losses, that is to say residual attenuation of the microwave even when the attenuator is at its minimum setting, due to its very presence. These insertion losses are due to the intrinsic characteristics of PIN diodes and may be as much as two to three decibels (dB). Such losses are unacceptable in certain practical applications, in particular at the output of a microwave transmission system.
It is also known to introduce a plate into the waveguide through a longitudinal slot parallel to the propagation direction of the microwave.
If the plate is made of a resistive material which absorbs the energy propagated in the guide, this energy will be reduced downstream of the plate relative to the propagation direction: this produces an attenuator.
The amplitude of the attenuation depends in each case on the size of the plate and the extent to which it forms an obstacle to the propagation of the wave in the guide. It is thus possible to produce a variable attenuator by modifying the depth to which the plate is inserted into the guide, or the distance separating it from the symmetry axis of the guide parallel to the propagation direction (where the amplitude of the field is greatest).
However, the attenuator is thus dependent on a large number of uncontrolled factors. The result of this, in particular, is that the attenuation is not a linear function of the position or insertion of the plate. For this reason, the positional slaving of this plate will have its gain vary as a function of the setpoint value. In certain cases, there is a risk of the slaving being slack, and in other cases there is a risk of it oscillating.